The ReDi Restaurant Reservation WordPress plugin before 21.0426 provides the functionality to let users make restaurant reservations. These reservations are stored and can be listed on an 'Upcoming' page provided by the plugin. An unauthenticated user can fill in the form to make a restaurant reservation. The form to make a restaurant reservation field called 'Comment' does not use proper input validation and can be used to store XSS payloads. The XSS payloads will be executed when the plugin user goes to the 'Upcoming' page, which is an external website https://upcoming.reservationdiary.eu/ loaded in an iframe, and the stored reservation with XSS payload is loaded.